Reflective stack with heat spreading layer

ABSTRACT

Reflective stacks including heat spreading layers are described. In particular, reflective stacks including polymeric multilayer reflectors. Heat spreading layers may include natural or synthetic graphite or copper.

BACKGROUND

Backlights for display devices are sometimes exposed to uneven andsignificant sources of heat. Sometimes the heat is from componentsinternal to the device and sometimes it is from an external source.Reflectors are used in displays to minimize absorptive losses and toimprove the gain of the displays in conjunction with recycling filmslike prism films and reflective polarizers. Conductive metals and carbon(graphite) are used to conduct and spread heat.

SUMMARY

In one aspect, the present disclosure relates to a reflective stack. Thereflective stack includes first polymeric multilayer reflector having amajor surface, a heat spreading layer disposed on the major surface ofthe polymeric multilayer reflector, and a second polymeric multilayerreflector disposed on the heat spreading layer opposite the firstpolymeric multilayer reflector. The first polymeric multilayer reflectorhas a first thickness and the second polymeric multilayer reflector hasa second thickness, and the first thickness and the second thickness arewithin 5% of each other.

In another aspect, the present disclosure relates to a reflective stack.The reflective stack includes a polymeric multilayer reflector having amajor surface, a heat spreading layer having at least one edge, the heatspreading layer disposed on the major surface of the polymericmultilayer reflector, and a polymeric film disposed on the heatspreading layer opposite the polymeric multilayer reflector. Thepolymeric multilayer reflector and the polymeric film are larger thanthe heat spreading layer such that, from a plan view, there is at leasta 0.1 mm border along a portion of the at least one edge that includesthe polymeric multilayer reflector and the polymeric film but no heatspreading layer, and the polymeric film is adhered directly to thepolymeric multilayer reflector at the border.

In yet another aspect, the present disclosure relates to a reflectivestack. The reflective stack includes a polymeric multilayer reflectorhaving a major surface, a first polymeric film disposed on the majorsurface of the polymeric multilayer reflector, a heat spreading layerhaving at least one edge, the heat spreading layer disposed on firstpolymeric film opposite the polymeric multilayer reflector, and a secondpolymeric film disposed on the heat spreading layer opposite thepolymeric multilayer reflector. The polymeric multilayer reflector andthe second polymeric film are larger than the heat spreading layer suchthat, from a plan view, there is at least a 0.1 mm border along aportion of the at least one edge that includes the polymeric multilayerreflector and the polymeric film but no heat spreading layer, and thesecond polymeric film is adhered directly to the polymeric multilayerreflector at the border.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation cross-section of a reflective stack.

FIG. 2 is an elevation cross-section of another reflective stack.

FIG. 3 is an elevation cross-section of another reflective stack.

FIG. 4 is an elevation cross-section of another reflective stack.

DETAILED DESCRIPTION

Backlights for displays for devices require the simultaneous powering ofseveral different components. In some cases, the powering of thesecomponents results in localized heat generation which may causeaccelerated or non-uniform wear of components, such as microprocessors,batteries, or other electronics. In some cases, the localized heatgeneration may make a device uncomfortable to hold if it is handheld orportable, may damage proximate heat sensitive components, may melt ordeform parts with poor thermal performance, and may shorten the usefullifetime of certain components. In some embodiments, it may be desirableto spread the heat generated by such components over a larger surfacearea.

FIG. 1 is an elevation cross-section of a reflective stack. Reflectivestack 100 includes polymeric film 110, adhesive 112, heat spreadinglayer 120, and polymeric multilayer reflector 130.

Polymeric multilayer reflector 130 may be any suitable size and shape,including any suitable thickness. Polymeric multilayer reflectors, suchas Enhanced Specular Reflector (ESR) (available from 3M Co., St. Paul,Minn.) are known and such films are commercially available.

Multilayer optical films, including multilayer reflectors have beendemonstrated by coextrusion of alternating polymer layers. See, e.g.,U.S. Pat. No. 3,610,729 (Rogers), U.S. Pat. No. 4,446,305 (Rogers etal.), U.S. Pat. No. 4,540,623 (Im et al.), U.S. Pat. No. 5,448,404(Schrenk et al.), and U.S. Pat. No. 5,882,774 (Jonza et al.). In thesepolymeric multilayer optical films, polymer materials are usedpredominantly or exclusively in the makeup of the individual layers.These may also be referred to as thermoplastic multilayer optical films.Such films are compatible with high volume manufacturing processes andcan be made in large sheets and roll goods. The description and examplesbelow relate to thermoplastic multilayer optical films.

A multilayer optical film includes individual microlayers havingdifferent refractive index characteristics so that some light isreflected at interfaces between adjacent microlayers. The microlayersare sufficiently thin so that light reflected at a plurality of theinterfaces undergoes constructive or destructive interference in orderto give the multilayer optical film the desired reflective ortransmissive properties. For multilayer optical films designed toreflect light at ultraviolet, visible, or near-infrared wavelengths,each microlayer generally has an optical thickness (a physical thicknessmultiplied by refractive index) of less than about 1 μm. Layers may bearranged generally as thinnest to thickest. In some embodiments, thearrangement of the alternating optical layers may vary substantiallylinearly as a function of layer count. These layer profiles may bereferred to as linear layer profiles. Thicker layers may be included,such as skin layers at the outer surfaces of the multilayer opticalfilm, or protective boundary layers (PBLs) disposed within themultilayer optical films, that separate coherent groupings (referred toherein as “packets”) of microlayers. In some cases, the protectiveboundary layer may be the same material as at least one of thealternating layers of the multilayer optical film. In other cases, theprotective boundary layer may be a different material, selected for itsphysical or rheological properties. The protective boundary layers maybe on one side or one both sides of an optical packet. In the case of asingle-packet multilayer optical film, the protective boundary layer maybe on one or both external surfaces of the multilayer optical film.Multilayer optical films may also include additional optical coatings orlayers (such as prisms, diffusers, or the like) or non-optical layersfor dimensional stability, warp resistance, impact protection, or thelike.

In some cases, the microlayers have thicknesses and refractive indexvalues providing a ¼-wave stack, i.e., arranged in optical repeat unitsor unit cells each having two adjacent microlayers of equal opticalthickness (f-ratio=50%), such optical repeat unit being effective toreflect by constructive interference light whose wavelength λ is abouttwice the overall optical thickness of the optical repeat unit. Otherlayer arrangements, such as multilayer optical films having 2-microlayeroptical repeat units whose f-ratio is different from 50%, or films whoseoptical repeat units include more than two microlayers, are also known.These optical repeat unit designs can be configured to reduce or toincrease certain higher-order reflections. See, e.g., U.S. Pat. No.5,360,659 (Arends et al.) and U.S. Pat. No. 5,103,337 (Schrenk et al.).Thickness gradients of the optical repeat units along a thickness axisof the film (e.g., the z-axis) can be used to provide a widenedreflection band, such as a reflection band that extends over the entirehuman visible region and into the near infrared so that as the bandshifts to shorter wavelengths at oblique incidence angles the microlayerstack continues to reflect over the entire visible spectrum. Thicknessgradients tailored to sharpen band edges, i.e., the wavelengthtransition between high reflection and high transmission, are discussedin U.S. Pat. No. 6,157,490 (Wheatley et al.).

In many applications, the reflection properties of a reflector may becharacterized in terms of “hemispheric reflectivity,” R_(hemi)(λ),meaning the total reflectivity of a component (whether a surface, film,or collection of films) when light (of a certain wavelength orwavelength range of interest) is incident on it from all possibledirections. Thus, the component is illuminated with light incident fromall directions (and all polarization states, unless otherwise specified)within a hemisphere centered about a normal direction, and all lightreflected into that same hemisphere is collected. The ratio of the totalflux of the reflected light to the total flux of the incident light forthe wavelength range of interest yields the hemispheric reflectivity,R_(hemi)(λ). Characterizing a reflector in terms of its R_(hemi)(λ) maybe especially convenient for backlight recycling cavities because lightis often incident on the internal surfaces of the cavity—whether thefront reflector, back reflector, or side reflectors—at all angles.Further, unlike the reflectivity for normal incident light, R_(hemi)(λ)is insensitive to, and already takes into account, the variability ofreflectivity with incidence angle, which may be very significant forsome components within a recycling backlight (e.g., prismatic films).

It is understood that for numerous electronic display applications usingbacklights, and that for backlights for general and specialty lightingapplications, it may be desirable for the reflector film forming thebacklight's backplane to have high reflectivity characteristics. Indeed,it is further understood that the hemispheric reflectivity spectrum,R_(hemi)(λ), strongly correlates with light output efficiency of abacklight; the higher the R_(hemi)(λ) value across the visible lightspectrum, the higher the output efficiency of the backlight. This isparticularly true for recycling backlights, where other optical filmsmay be configured over the backlight exit aperture to provide collimatedor polarized light output from the backlight.

Further details of multilayer optical films and related designs andconstructions are discussed in U.S. Pat. No. 5,882,774 (Jonza et al.)and U.S. Pat. No. 6,531,230 (Weber et al.), PCT Publications WO 95/17303(Ouderkirk et al.) and WO 99/39224 (Ouderkirk et al.), and thepublication entitled “Giant Birefringent Optics in Multilayer PolymerMirrors”, Science, Vol. 287, March 2000 (Weber et al.).

The reflective and transmissive properties of multilayer optical filmare a function of the refractive indices of the respective microlayersand the thicknesses and thickness distribution of the microlayers. Eachmicrolayer can be characterized at least in localized positions in thefilm by in-plane refractive indices n_(x), n_(y), and a refractive indexn_(z) associated with a thickness axis of the film. These indicesrepresent the refractive index of the subject material for lightpolarized along mutually orthogonal x-, y-, and z-axes, respectively.For ease of explanation in the present description, unless otherwisespecified, the x-, y-, and z-axes are assumed to be local Cartesiancoordinates applicable to any point of interest on a multilayer opticalfilm, in which the microlayers extend parallel to the x-y plane, andwherein the x-axis is oriented within the plane of the film to maximizethe magnitude of Δn_(x). Hence, the magnitude of Δn_(y) can be equal toor less than—but not greater than—the magnitude of Δn_(x). Furthermore,the selection of which material layer to begin with in calculating thedifferences Δn_(x), Δn_(y), Δn_(z) is dictated by requiring that Δn_(x)be non-negative. In other words, the refractive index differencesbetween two layers forming an interface are Δn_(j)=n_(1j)−n_(2j), wherej=x, y, or z and where the layer designations 1,2 are chosen so thatn_(1x)≥n_(2x), i.e., Δn_(x)≥0.

In practice, the refractive indices are controlled by judiciousmaterials selection and processing conditions. A multilayer film is madeby co-extrusion of a large number, e.g. tens or hundreds of layers oftwo alternating polymers A, B, sometimes followed by passing themultilayer extrudate through one or more multiplication die, and thenstretching or otherwise orienting the extrudate to form a final film.The resulting film is typically composed of many hundreds of individualmicrolayers whose thicknesses and refractive indices are tailored toprovide one or more reflection bands in desired region(s) of thespectrum, such as in the visible or near infrared. To achieve highreflectivities with a reasonable number of layers, adjacent microlayerstypically exhibit a difference in refractive index (Δn_(x)) for lightpolarized along the x-axis of at least 0.05. In some embodiments,materials are selected such that the difference in refractive index forlight polarized along the x-axis is as high as possible afterorientation. If the high reflectivity is desired for two orthogonalpolarizations, then the adjacent microlayers also can be made to exhibita difference in refractive index (Δn_(y)) for light polarized along they-axis of at least 0.05.

Polymeric multilayer optical films as described herein may be highlyreflective; for example, they may reflect more than 95% or 99% or even99.5% of visible light, as measured at normal incidence. Visible lightmay be characterized as wavelengths between 400 nm and 700 nm, or insome cases between 420 nm and 700 nm. Further, polymeric multilayeroptical films as described herein may be thin—in some cases, thinnerthan 100 μm, 85 μm, or 65 μm, 50 μm, 35 μm, or even 32 μm. Inembodiments where the polymeric multilayer optical film includes a thirdoptical packet, the film may be thinner than 165 μm.

Skin layers are sometimes added which occurs after the feedblock butbefore the melt exits the film die. The multilayer melt is then castthrough a film die onto a chill roll in the conventional manner forpolyester films, upon which it is quenched. The cast web is thenstretched in different ways to achieve birefringence in at least one ofthe optical layers, producing in many cases either a reflectivepolarizer or mirror film, as has been described in, for example, U.S.Patent Publication No. 2007/047080 A1, U.S. Patent Publication No.2011/0102891 A1, and U.S. Pat. No. 7,104,776 (Merrill et al.).

Heat spreading layer 120 may be any suitable material and may be anysuitable size and shape. In some embodiments, heat spreading layer 120is undersized compared to polymeric multilayer reflector 130. Heatspreading layer 120 may be formed from conductive metals, such as copperor silver, or it may be formed from other suitable materials such asceramics like boron nitride. Other heat spreading layer options includesynthetic and natural graphite. Isotropic heat spreading materials maybe used in some embodiments, and in some embodiments, anisotropicmaterials may be desired. Anisotropic materials may, for example, bevery conductive in the X- and Y-directions (i.e., in plane) but not veryconductive in the Z-direction. In some embodiments, heat spreading layer120 includes multiple layers of heat spreading material. Heat spreadinglayer 120 may have any suitable thickness: in some embodiments, heatspreading layer 120 is thinner than 50 μm, or thinner than 30 μm. Heatspreading layer 120 may be a twentieth the area of polymeric multilayerreflector 130, a tenth, eighth, sixth, fifth, third, half, two-thirds,three-quarters, or even 90%, 95%, or 99% the area of polymericmultilayer reflector 130. In some embodiments, heat spreading layer 120may be mostly a regular shape, such as a rectangular or square, but mayinclude one or more tabs.

Heat spreading layer 120 is disposed on, but not adhered to polymericmultilayer reflector 120. This is signified by the dashed linesconnecting heat spreading layer 120 to polymeric multilayer reflector130.

Polymeric film 110 is adhered to heat spreading layer 120 and polymericmultilayer reflector 120 via adhesive 112. Adhesive 112 may be anysuitable adhesive and have any suitable thickness. Adhesive 112 isviscous enough so that it does not flow around and fill the spacebetween heat spreading layer 120 and polymeric multilayer reflector 130.In other words, adhesive 112 joins protective layer 110 and heatspreading layer 120, and protective layer 110 and polymeric multilayerreflector 130, but not heat spreading layer 120 and polymeric multilayerreflector 120. In some embodiments, adhesive 112 may be a pressuresensitive adhesive. In some embodiments, adhesive 112 may be a UVcurable adhesive. In some embodiments, adhesive 112 is a hot meltadhesive.

Polymeric film “edge seals” heat spreading layer 120. Accordingly,polymeric film 110 is oversized compared to heat spreading layer 120. Insome embodiments, polymeric film 120 is adhered to polymeric multilayerreflector 130 directly via adhesive 112, without heat spreading layer120 in between. FIG. 1 is merely a cross-section, and therefore showsthat heat spreading layer 120 is edge sealed only along two edges. Insome embodiments, heat spreading layer 120 is edge sealed along threeedges, along four edges, or more, depending on the overall shape of heatspreading layer 120. In some embodiments, heat spreading layer 120 hasone or more tabs and these tabs extend beyond polymeric film 110.Polymeric film 110 may be any suitable shape and thickness as well, and,despite its illustration in FIG. 1 as being sharply bent at the edges,may have any number of curved or straight parts. In some embodiments,polymeric film 110 may be scored to provide for easier bending. In someembodiments, polymeric film 110 is larger than both heat spreading layer120 and polymeric multilayer reflector 130 such that when heat spreadinglayer 120 is edge sealed, there is, from a plan view, an exposed portionof polymeric film 110 extending beyond the polymeric multilayerreflector. In some embodiments, the polymeric film may be a continuousroll and the polymeric multilayer reflector and/or the heat spreadinglayer may be discontinuously arranged, such that the polymeric film maybe wound as a roll with a plurality of discontinuous reflective stacksdisposed thereon.

Polymeric film 110 may be any suitable material. In some embodiments,polymeric film 110 may be a polymeric material, such as polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), poly(methylmethacrylate) (PMMA), or polycarbonate (PC). In some embodiments,polymeric film 110 may be a polymeric multilayer reflector. In someembodiments, polymeric film 110 may be a polymeric multilayer reflectorwith the same thickness and the same configuration as polymericmultilayer reflector 130. In some embodiments, polymeric film 110 is apolymeric multilayer reflector that has a thickness that is within 10%,or within 5% of the thickness of polymeric multilayer reflector 130.

The direct interface between heat spreading layer 120 and polymericmultilayer reflector 130 may be beneficial from the standpoint ofpromoting better heat spreading, as there are no intervening layers. Atthe same time, the edge sealing of heat spreading layer 120 by polymericfilm 110 prevents contamination of other device electronics orcomponents as certain heat spreading layer materials, such as graphite,tend to shed or lose material over time, and this material can migrateinto the electronics or components.

Polymeric film 110 may be, however, perforated or have at least one holeor punchout. This punchout may be through heat spreading layer 120 aswell. Likewise, polymeric multilayer reflector 130 may also beperforated or have at least one hole or punchout. This may help withthermal expansion and overall weight, and may also help with radiofrequency tranparency. Suitable balances between perforation andcontamination prevention may be determined by the specific application.

FIG. 2 is an elevation cross-section of another reflective stack.Reflective stack 200 includes polymeric film 210, first adhesive 212,heat spreading layer 220, second adhesive 222, and polymeric multilayerreflector 230. Reflective stack 200 of FIG. 2 is similar to reflectivestack 100 of FIG. 1 except heat spreading layer 220 is adhered topolymeric multilayer reflector 230 with second adhesive 222 as well asbeing edge sealed by polymeric film 210 via first adhesive 212.

FIG. 3 is an elevation cross-section of another reflective stack.Reflective stack 300 includes first polymeric film 310, first adhesive312, heat spreading layer 320, second adhesive 322, second polymericfilm 340, third adhesive 342, and polymeric multilayer reflector 330.Reflective stack 300 is similar to reflective stack 200 of FIG. 2 exceptthe reflective stack further includes a second polymeric film 340disposed on heat spreading layer 320, and the second polymeric film isadhered to the polymeric multilayer reflector, via third adhesive 342.Second polymeric film 340 is adhered to heat spreading layer 320 viasecond adhesive 322. Even though heat spreading layer 320 is disposedbetween first polymeric film 310 and second polymeric film 340, heatspreading layer 320 is not fully enclosed by or sealed between thepolymeric films. Second polymeric film 340 may be any suitable polymericfilm, and may be similar to or different from first polymeric film 310.In some embodiments, the second polymeric film may be disposed on theopposite side as shown in FIG. 3; that is, first polymeric film 310 isadhered to second polymeric film 340 via first adhesive 312.

FIG. 4 is an elevation cross-section of another reflective stack.Reflective stack 400 includes first polymeric film 410, first adhesive412, heat spreading layer 420, second adhesive 422, second polymericfilm 440, and polymeric multilayer reflector 430. Reflective stack 400is similar to reflective stack 300 of FIG. 3 except second polymericfilm 440 is disposed on but not adhered to polymeric multilayerreflector 430. As for reflective stack 300 in FIG. 3, in someembodiments, second polymeric film 440 may be disposed on the oppositeside of heat spreading layer 420, such that heat spreading layer 420 maybe disposed on but not adhered to polymeric multilayer reflector 430.

Laminated rolls of sealed graphite pouches are described in U.S. Pat.No. 8,563,104 (Rappoport et al.) but describe roll-to-roll processesthat require expensive and wasteful conversion, in particular, thenecessitation of the disposal of a large percentage of often-expensivemultilayer polymeric reflector. Further, Rappoport et al. requires extramaterial for carrier layers and the pouches of graphite to be fullysealed, as opposed to edge sealing or otherwise not fully sealing thegraphite in a pouch. Accordingly, heat spreading, warping, and cosmeticquality from print through may be poorer than the performance ofembodiments described herein

Embodiments described herein may have one or more advantages over otherheat spreading reflective stacks. First, reflective stacks describedherein may be very thin and may still provide excellent thermalspreading properties. In some embodiments, they may be thinner than 200μm, 150 μm, 130 μm, 100 μm, or even 50 μm. Reflective stacks describedherein may also resist high heat and/or high humidity exposure orcycling in that they do not permanently curl or deform. Reflectivestacks described herein may also help mitigate “print through” effects,where either dents in or surface roughness of the heat spreading layerpresses into the polymeric multilayer reflector and causes visiblecosmetic defects in the reflector.

EXAMPLES

Samples for these examples were evaluated using the following tests.Three variations of environmental testing were used. The first was athermal shock (TS) test where the sample was placed in an oven andexposed for 24 hours to cycles of 1 hour at 40 degrees C. alternatingwith 1 hour at 85 degrees C. A second test was a high temperature (HT)test where the sample placed in a dry oven (at about 3% relativehumidity) and exposed to 85 degrees C. for 24 hours. A third test was ahigh temperature, high humidity (HTHH) test where the sample was placedin an oven and exposed to 65 degrees C. and 95% relative humidity for 24hours. After the environmental testing, the sample was placed in achamber for reconditioning at 22 degrees C. and 50% relative humidity.

Each sample was evaluated for edge part curl before environmentaltesting and after reconditioning. To do this, the rectangular sample wasplaced on a flat surface and a ruler was used to measure the distancesthat the edges of the four sides of the sample rose above the flatsurface. Average edge part curl was computed as the arithmetic averageof the four edge part curl measurements. Change in average edge partcurl was the difference between the average edge part curl afterreconditioning and the average initial edge part curl.

A visual rating was also assigned to each sample as an assessment of theappearance of the reflective surface after environmental testing. Arating of “excellent” meant that the surface had an acceptable visualappearance with a few small flaws appropriate for low diffusionbacklight systems, “good” meant that small flaws were more numerous, and“marginal” meant that the sample had an appearance best suited for highdiffusion backlight designs

Select samples were also evaluated to determine edge fidelity by howwell the graphite sheet was centered on the protective sheet. Using aruler, the widths of the border were measured on the midpoint of eachside of the sample.

Example 1-1. A sample was prepared as follows. Using a rotary convertingprocess a stack of three films was assembled. The bottom film was a 2micron thick PET protective sheet of dimensions 65 mm by 115 mm with a 5micron thick transfer adhesive (3M 82600 adhesive from 3M Company, St.Paul Minn.) applied to one side. Next, a 25 micron thick graphite sheet(AvCarb HS-025 available from AvCarb Material Solutions, Lowell Mass.)was positioned on the lower film so that it left about a 1 mm borderaround the perimeter of the graphite sheet. (The graphite sheet haddimensions approximately 64 mm by 114 mm.) Then a 32 micron thickreflective film (ESR2 from 3M Company, St. Paul Minn.) having the samedimensions as the PET sheet was attached above the graphite film withthe reflective side facing away from the graphite film. It was held inplace by the adhesive around the border of the graphite film.

Edge part curl of the sample was evaluated before environmental testingand after reconditioning. The environmental test was thermal shock (TS)testing. The visual appearance of the sample was evaluated afterreconditioning. Results are recorded in Table 1. The widths of theborder were measured to be 0.68, 0.56, 0.70 and 0.84 mm.

Example 1-2. A second sample was made and tested as in Example 1-1.Results are shown in Table 1. Border widths were measured to be 1.63,0.47, 0.0, and 0.96 mm.

Example 1-3. A sample was made as in Example 1-1. The environmentaltesting was high temperature (HT) testing. Edge part curl was measuredbefore environmental testing and following reconditioning. Visualappearance was also evaluated after reconditioning. Results are reportedin Table 1. Border widths were measured to be 1.51, 0.91, 0.35, and 0.62mm.

Example 1-4. Another sample was made and tested as in Example 1-3.Results are reported in Table 1. Measured border widths were 1.43, 0.61,0.39 and 0.77 mm.

Example 1-5. A sample was made as in Example 1-1. The environmentaltesting was high temperature—high humidity (HTHH) testing. Edge partcurl was measured before environmental testing and followingreconditioning. Results are reported in Table 1. Border widths were0.41, 0.60, 1.17 and 0.91 mm.

Example 1-6. Another sample was made and tested as in Example 1-5.Results are reported in Table 1. Measured border widths were 0.95, 0.96,0.68 and 0.55 mm.

Example 2-1. A sample was prepared as follows. Using a rotary convertingprocess a stack of films was assembled. The bottom film was a 1 mil (25micron) thick PET protective sheet of dimensions 65 mm by 115 mm with a10 micron thick transfer adhesive (3M 84401 adhesive from 3M Company,St. Paul Minn.) applied to one side. Next, a sheet of 55400P0 (fromGrafTech International Holdings, Independence OH,) was positioned withthe graphite film facing the lower film and leaving about a 1 mm borderaround the perimeter. (The 55400P0 sheet had dimensions approximately 64mm by 114 mm.) Next a second adhesive layer (also 10 micron thick 3M84401 adhesive) was applied over the 55400P0 sheet. Then a 65 micronthick reflective film (ESR from 3M Company) having the same dimensionsas the bottom PET sheet was attached above the graphite film with thereflective side facing away from the graphite film.

Edge part curl of the sample was evaluated before environmental testingand after reconditioning. The environmental test was thermal shock (TS)testing. The visual appearance of the sample was evaluated afterreconditioning. Results are recorded in Table 1.

Example 2-2. A second sample was made and tested as in Example 2-1.Results are recorded in Table 1.

Example 2-3. A sample was made as in Example 2-1, but tested using thehigh temperature (HT) test. Results are recorded in Table 1.

Example 2-4. A second sample was made and tested as in Example 2-3.Results are recorded in Table 1.

Example 2-5. A sample was made as in Example 2-1, but tested using thehigh temperature, high humidity (HTHH) test. Results are recorded inTable 1.

Example 2-6. A second sample was made and tested as in Example 2-5.Results are recorded in Table 1.

Example 3-1. A sample was prepared as follows. Using a rotary convertingprocess a stack of three films was assembled. The bottom film was a 1mil (25 micron) thick PET protective sheet of dimensions 65 mm by 115 mmwith a 10 micron thick transfer adhesive (3M 84401 adhesive) applied toone side. Next, 25 micron thick graphite sheet (from AvCarb MaterialSolutions, Lowell Mass.) was positioned on the lower film so that itleft about a 1 mm border around the perimeter. (The graphite sheet haddimensions approximately 64 mm by 114 mm.) 3M 84401 adhesive (10 micronsthick) was applied over the graphite film, and then a 65 micron thickreflective film (ESR from 3M Company, St. Paul Minn.) having the samedimensions as the bottom PET sheet was attached to the graphite filmwith the reflective side facing away from the graphite film.

Edge part curl of the sample was evaluated before environmental testingand after reconditioning. The environmental test was thermal shock (TS)testing. The visual appearance of the sample was evaluated afterreconditioning. Results are recorded in Table 1.

Example 3-2. A second sample was made and tested as in Example 3-1.Results are reported in Table 1.

Example 3-3. A sample was made as in Example 3-1, but tested using thehigh temperature (HT) test. Results are recorded in Table 1.

Example 3-4. A second sample was made and tested as in Example 3-3.Results are reported in Table 1.

Example 3-5. A sample was made as in Example 3-1, but tested using thehigh temperature, high humidity (HTHH) test. Results are recorded inTable 1.

Example 3-6. A second sample was made and tested as in Example 3-5.Results are reported in Table 1.

Example 4-1. A sample was prepared as follows. Using a rotary convertingprocess a stack of four films was assembled. The bottom film was a 2micron thick PET protective sheet of dimensions 65 mm by 115 mm with a 5micron thick transfer adhesive (3M 82600 adhesive) applied to one side.Next, a 40 micron thick graphite sheet with a protective tape (SS400P0from GrafTech International Holdings, Independence OH) was positioned onthe lower film so that it left about a 1 mm border around the perimeterof the graphite sheet (The graphite sheet had dimensions approximately64 mm by 114 mm). Next a 65 micron thick reflective film (ESR from 3MCompany, St. Paul Minn.) having the same dimensions as the bottom PETsheet was attached above the second protective sheet with the reflectiveside facing away from the graphite film. It was held in place by theadhesive around the border of the graphite film.

Edge part curl of the sample was evaluated before environmental testingand after reconditioning. The environmental test was high temperature(HT) testing. The visual appearance of the sample was evaluated afterreconditioning. Results are recorded in Table 1.

Example 4-2. A second sample was made and tested as in Example 4-1.Results are recorded in Table 1.

Example 4-3. A third sample was made and tested as in Example 4-1.Results are recorded in Table 1.

Example 4-4. A sample was made as in Example 4-1 but tested usingthermal shock (TS) testing. Results are recorded in Table 1.

Example 4-5. A second sample was made and tested as in Example 4-4.Results are recorded in Table 1.

Example 4-6. A third sample was made and tested as in Example 4-4.Results are recorded in Table 1.

Example 5-1. A sample was prepared as follows. Using a rotary convertingprocess a stack of three films was assembled. The bottom film was a 65micron thick reflective film (ESR from 3M Company) of dimensions 65 mmby 115 mm with a 5 micron thick adhesive (3M 84400 adhesive) applied tothe non-reflective side. Next, a 40 micron thick graphite sheet with aprotective tape (SS400P0 from GrafTech International Holdings,Independence OH) was positioned on the lower film so that it left abouta 1 mm border around the perimeter of the graphite sheet. (The graphitesheet had dimensions approximately 64 mm by 114 mm.) Next an 80 micronthick reflective film (ESR80v2 from 3M Company) having the samedimensions as the bottom ESR sheet was attached above the graphite sheet(again using 5 micron thick 3M 84400 adhesive) with the reflective sidefacing away from the graphite film.

Edge part curl of the sample was evaluated before environmental testingand after reconditioning. The environmental test was thermal shock (TS)testing. The visual appearance of the sample was evaluated afterreconditioning. Results are recorded in Table 1.

Example 5-2. A second sample was prepared and tested as in Example 5-1.Results are recorded in Table 1.

Example 5-3. A sample was prepared as in Example 5-1, but tested usingthe high temperature (HT) test. Results are recorded in Table 1.

Example 5-4. A second sample was prepared and tested as in Example 5-3.Results are recorded in Table 1.

Example 5-5. A sample was prepared as in Example 5-1, but tested usingthe high temperature—high humidity (HTHH) test. Results are recorded inTable 1.

Example 5-6. A second sample was prepared and tested as in Example 5-5.Results are recorded in Table 1.

TABLE 1 Environmental Change in Visual Test average edge appearanceExample Condition part curl (mm) rating Example 1-1 TS 0.25 Good Example1-2 TS 0.5 Good Example 1-3 HT 0.625 Excellent Example 1-4 HT −0.125Excellent Example 1-5 HTHH 3.25 Good Example 1-6 HTHH 1.75 Good Example2-1 TS 13.5 Marginal Example 2-2 TS 14.5 Marginal Example 2-3 HT 14Marginal Example 2-4 HT 14 Marginal Example 2-5 HTHH −1.5 MarginalExample 2-6 HTHH −1.5 Marginal Example 3-1 TS 15 Excellent Example 3-2TS 16.25 Excellent Example 3-3 HT 3.25 Excellent Example 3-4 HT 2.125Excellent Example 3-5 HTHH 2.75 Excellent Example 3-6 HTHH 4 ExcellentExample 4-1 HT −3.5 Good Example 4-2 HT 1.75 Good Example 4-3 HT −1.5Good Example 4-4 TS −5.5 Good Example 4-5 TS −3.25 Good Example 4-6 TS−4 Good Example 5-1 TS 1.5 Marginal Example 5-2 TS 3 Marginal Example5-3 HT 1.75 Marginal Example 5-4 HT 3.25 Marginal Example 5-5 HTHH 2Marginal Example 5-6 HTHH 3 Marginal

The following are exemplary embodiments according to the presentdisclosure:

Item 1. A reflective stack, comprising:

-   -   a first polymeric multilayer reflector having a major surface;    -   a heat spreading layer disposed on the major surface of the        polymeric multilayer reflector; and    -   a second polymeric multilayer reflector disposed on the heat        spreading layer opposite the first polymeric multilayer        reflector;    -   wherein the first polymeric multilayer reflector has a first        thickness and the second polymeric multilayer reflector has a        second thickness, and the first thickness and the second        thickness are within 5% of each other.

Item 2. A reflective stack, comprising:

-   -   a polymeric multilayer reflector having a major surface;    -   a heat spreading layer having at least one edge, the heat        spreading layer disposed on the major surface of the polymeric        multilayer reflector; and    -   a polymeric film disposed on the heat spreading layer opposite        the polymeric multilayer reflector;    -   wherein the polymeric multilayer reflector and the polymeric        film are larger than the heat spreading layer such that, from a        plan view, there is at least a 0.1 mm border along a portion of        the at least one edge that includes the polymeric multilayer        reflector and the polymeric film but no heat spreading layer;        and    -   wherein the polymeric film is adhered directly to the polymeric        multilayer reflector at the border.

Item 3. The reflective stack of item 2, wherein the at least one edgeincludes four edges, and, from a plan view, there is at least a 0.1 mmborder along two of the four edges that includes the polymericmultilayer reflector and the polymeric film but no heat spreading layer.

Item 4. The reflective stack of item 3, wherein, from a plan view, thereis at least a 0.1 mm border along three of the four edges that includesthe polymeric multilayer reflector and the polymeric film but no heatspreading layer.

Item 5. The reflective stack of item 2, wherein the heat spreading layeris not adhered to the major surface of the polymeric multilayerreflector.

Item 6. The reflective stack of item 2, wherein the heat spreading layeris adhered to the major surface of the polymeric multilayer reflector.

Item 7. The reflective stack of item 2, wherein the heat spreading layeris adhered to the polymeric film.

Item 8. The reflective stack of item 2, wherein the heat spreading layerincludes natural graphite.

Item 9. The reflective stack of item 2, wherein the heat spreading layerincludes synthetic graphite.

Item 10. The reflective stack of item 2, wherein the heat spreadinglayer includes copper.

Item 11. A backlight, comprising the reflective stack of item 2.

Item 12. A display, comprising the reflective stack of item 2.

Item 13. A reflective stack, comprising:

-   -   a polymeric multilayer reflector having a major surface;    -   a first polymeric film disposed on the major surface of the        polymeric multilayer reflector;    -   a heat spreading layer having at least one edge, the heat        spreading layer disposed on first polymeric film opposite the        polymeric multilayer reflector; and    -   a second polymeric film disposed on the heat spreading layer        opposite the polymeric multilayer reflector;    -   wherein the polymeric multilayer reflector and the second        polymeric film are larger than the heat spreading layer such        that, from a plan view, there is at least a 0.1 mm border along        a portion of the at least one edge that includes the polymeric        multilayer reflector and the polymeric film but no heat        spreading layer; and    -   wherein the second polymeric film is adhered directly to the        polymeric multilayer reflector at the border.

Item 14. The reflective stack of item 13, wherein the first polymericfilm is disposed such that it is closer to the second polymeric filmthan the polymeric multilayer reflector.

Item 15. The reflective stack of item 13, wherein the first polymericfilm is disposed such that it is closer to the second polymeric filmthan the polymeric multilayer reflector.

Item 16. The reflective stack of item 13, wherein at least one of thefirst polymeric film or the second polymeric film has at least oneperforation.

Item 17. The reflective stack of item 13, wherein the at least one edgeincludes four edges, and, from a plan view, there is at least a 0.1 mmborder along two of the four edges that includes the polymericmultilayer reflector and the second polymeric film but no heat spreadinglayer.

Item 18. The reflective stack of item 17, wherein, from a plan view,there is at least a 0.1 mm border along three of the four edges thatincludes the polymeric multilayer reflector and the polymeric film butno heat spreading layer.

Item 19. The reflective stack of item 13, wherein the heat spreadinglayer includes graphite.

Item 20. A roll of film, comprising:

-   -   a plurality of reflective stacks as in item 13;        wherein between two adjacent reflective stacks of the plurality        of reflective stacks, the second polymeric film is continuous        but the polymeric multilayer reflector is discontinuous.

Descriptions for elements in figures should be understood to applyequally to corresponding elements in other figures, unless indicatedotherwise. The present invention should not be considered limited to theparticular examples and embodiments described above, as such embodimentsare described in detail in order to facilitate explanation of variousaspects of the invention. Rather, the present invention should beunderstood to cover all aspects of the invention, including variousmodifications, equivalent processes, and alternative devices fallingwithin the scope of the invention as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A reflective stack, comprising: a first polymericmultilayer reflector having a major surface; a heat spreading layerdisposed on the major surface of the polymeric multilayer reflector; anda second polymeric multilayer reflector disposed on the heat spreadinglayer opposite the first polymeric multilayer reflector; wherein thefirst polymeric multilayer reflector has a first thickness and thesecond polymeric multilayer reflector has a second thickness, and thefirst thickness and the second thickness are within 5% of each other.